Protein Association and Nucleocytoplasmic Distribution: An Experimental and Modeling Study

Protein abundance can shape signaling not only by changing how much of a signaling component is available, but also by altering the molecular states that component occupies. Here, we examined this principle in the Smad2/3 branch of the TGF-β superfamily. Using lentivirally expressed fluorescent R-Smads in HEK293T cells, we found that nuclear Smad2 accumulation increased disproportionately across the observed range of Smad2-GFP abundance. A minimal compartmental model reproduced this abundance-dependent behavior when concentration-dependent R-Smad dimerization was included, whereas a reduced model containing monomeric shuttling alone produced an approximately proportional response. FRET measurements further showed receptor-sensitive proximity between Smad2-GFP and Smad3-RFP, providing independent evidence for R-Smad association in the experimental system. A comparison nucleocytoplasmic shuttling system displayed a more nearly proportional abundance-localization relationship, and increasing the intracellular Smad pool preserved the characteristic Smad localization pattern. Taken together, these observations support a mechanism in which concentration-dependent R-Smad association links protein abundance to spatial redistribution between cytoplasm and nucleus. This provides a physical route by which variation in signaling-protein abundance can be converted into nonlinear intracellular localization dynamics.